The mechanism described briefly in the paragraph above is well-known in the art. This mechanism is fitted to a timepiece, called a large date timepiece where the day of month appears through a large aperture arranged in the timepiece dial. The date then appears in larger dimensions than if it were printed on a single ring making one revolution less and moving by approximately 11.5 degrees per day.
The first and second indicators respectively indicating the units and tens of the day of the month are driven by a program wheel set which is described in document WO 01/77756. This wheel set is arranged to drive the first indicator at the end of every day by one step except at the end of the thirty first day when it is not driven, and the second indicator by one step at the end of the ninth, nineteenth, twenty-ninth and thirty first days of the month.
This program wheel set carries a day of the month wheel having thirty one teeth which is driven in a known manner by the beak of a lever controlled in turn either by the drive mechanism of the timepiece, or by a manually actuated pusher when a need to correct of the date is felt. In both cases, the change of date takes place when the beak of the lever—controlled manually or by the driving mechanism—penetrates between two teeth of the day of the month wheel and causes the latter to advance by one step.
Once the function is completed and due to the effect of a return spring acting on the lever, the beak is extracted from the wheel toothing.
This constructive arrangement has a major disadvantage, that of the risk of one of the indicators racing. Indeed, as the correction of the date is carried out by means of a pusher and as the force exerted manually on this pusher is not constant but depends on the strength with which the latter is pushed, the day of the month wheel and with it the program wheel set which is connected thereto can drive either the tens indicator, or the units indicator, as a result of said indicators, own inertia beyond the desired swing angle of a single step, consequently a progression of two units or only one unit can occur. For the tens indicator this danger exists if a correction is made the ninth, nineteenth, twenty-ninth and thirty-first day of the month and for the units indicator if a correction is made the thirtieth day of the month. This necessarily results in a loss of synchronization between the two indicators, and this synchronization can be restored only by opening the timepiece to restore order.
To avoid the disadvantages described in the paragraph above, at least at the time of the passage from the twenty-ninth to the thirtieth then from the thirtieth to the first day of the next month, document WO 01/77756 cited above implements a ratchet and two pins which prevent from one or other of the indicators racing. This system is relatively complicated and requires additional parts thus increasing the price of the timepiece.